Hex nuts and bolts are used in numerous applications to hold together temporary and permanent workpiece constructions. A bolt is a shaft with a helical groove on its surface, called the “thread.” A workpiece is an item that requires immobility with respect to another item, typically another workpiece. To ensure a stable workpiece, the bolt is placed through a hole in the workpiece. The workpiece hole may be threaded with a complimentary helix to mate with the bolt. Otherwise, the workpiece hole is not threaded.
Whether the workpiece hole is threaded or not threaded, the bolt must be held in place. Typically, this is done through a bolt head and a nut. The bolt head is attached to one end of the bolt. The nut is an object containing a hole with a complimentary threaded helix to mate with the bolt hole. Both the bolt head and nut are wider than the bolt shaft, preventing each from falling into the workpiece hole. Alternatively, two nuts are used to hold the bolt in place. Either way, both bolt head and nut must be shaped in order to allow for tightening to keep the workpiece assembly in place. Standard nuts and bolt heads typically have hexagonal shapes. An additional bolt type is a socket head bolt, or an “Allen™” head bolt. Socket head bolt heads are also wider than the bolt shaft but instead of being hexagonal in shape, are typically circular in shape and contain a recess which is hexagonal in shape. Socket head bolts perform functions interchangeable with standard bolts.
Tightening occurs by applying torque to the nut so that the nut turns along the bolt shaft until the nut reaches the workpiece assembly, thus creating a bolted workpiece assembly. Upon reaching the workpiece, additional tightening of the nut induces a tensile load in the associated bolt. This tensile load is often referred to as pre-load, or live load. Pre-load, increases the integrity of the workpiece assembly's bolted joint. Additionally, side loads, such as torque, may be induced in the bolt shaft. Side loading is typically undesirable. Bolted assemblies can become unintentionally loose by being subject to internal and external forces. For example, over time, workpieces may become very slightly indented or deformed due to the constant force applied to it through the nut. Or, fluctuation of external temperatures may create expansion and retraction of the material creating a loosening effect in the bolted workpiece assembly. Also, loosening may occur through vibration.
To lessen these loosening effects, washers are employed in the assembly. Washers are placed on the bolt prior to nut placement. Typical washers are thin disks with an unthreaded hole, usually in the middle. Washers limit indentation of a workpiece material by spreading the force applied to the workpiece over a greater surface area, thereby lowering the pressure applied on the workpiece surface. Additionally, washers known as Belleville washers help prevent bolted workpiece assemblies from loosening due to thermal expansion or vibration. Belleville washers are conical-shaped washers which act as springs. A nut is tightened onto the Belleville washer, flattening the washer. If the nut loosens slightly, the desired pre-load, or a substantial measure thereof, is still applied to the bolted assembly, through the biasing force applied by the Belleville washer.
Although Belleville washers provide a constant pre-load on bolted assemblies, they do not solve other problems inherent in high load applications. For example, bolted workpiece assemblies that employ Belleville washers are sill subject to side loading. As indicated above, side loading is any non-axial force applied to the bolt such as torque. By definition, torque is a rotational force. Since torque is required to tighten the nut against the Belleville washer, the torque applied to the outer diameter of the nut is transferred to the nut's helical thread on the inside of the nut as the nut turns around the bolt's thread. The torque is further transferred to the bolt as the nut's threads turn against the bolt's threads.
Side loading creates a possibility of bolt shearing during tightening, which occurs when the bolt's maximum tensile strength is exceeded. Additionally, residual side loading is often retained in the bolt, and if high enough, can weaken the bolt or reduce the ability of the bolt to absorb stresses that may incident on it and the associated work piece assembly.
Other types of devices besides Belleville washers that supply a constant pre-load or help maintain an initial level of preload also have inherent disadvantages. One type of device is a lock-nut, also referred to a jam nut. Although lock-nuts provide a locking mechanism on the bolt preventing movement or loosening of the lock nut or associated tightening nut on the bolt thereby helping to preserve the initially induced pre-load, these devices do nothing to prevent side loading of the bolt during tightening. Further, since none of the pre-load is stored in the lock nut unlike Belleville washers, the bolt will rapidly unload with any loosening of the nuts relative to the bolt.
Known prior art devices, such as products offered by Hytorc, a division Unex Corporation, of Mahwah, N.J., that provide for a constant pre-load without inducing side loading utilize multiple parts. Accordingly, they are very expensive compared to jam nuts and/or Belleview washers. Further, because they operate through a portion of the device shearing relative to another portion once a certain desired level of loading is achieved, they cannot be re-used.